Category Archives: Chapter 9 – Reaction Kinetics

Terminology and Concepts: Reaction Kinetics

Rate of reaction is the change of concentration of a reactant or a product per unit time (seconds / minutes).

Rate of reaction = rate of increase of the concentration or amount of the product / Rate of reaction = rate of decrease of the concentration or amount of the reactant.

Average rate is the change in concentration of a substance (reactant or product) over a fixed time interval.

Instantaneous rate is the rate of the reaction at a specific time (the steeper the slope, the higher the instantaneous rate).

Tangent is zero, the rate is zero / reaction has stopped.

Rate Equation or Rate Law is the rate of a reaction which is affected by the reactants concentration.

Rate = k [reactant]n , k is rate constant

————————————————————————————————————————————————–

Nucleophile – Lewis bases / a species that attacks a positively-charged (electron deficient) carbon atom by donating an electron-pair to form a dative covalent bond.

Electrophile – Lewis acids / a species that attacks a negatively-charged (electron rich) carbon atom by accepting an electron-pair to form a dative covalent bond.

Important Notes in Organic Chemistry:

If the concentration of the nucleophile influences the rate of the reaction which means the rate determining step involves the nucleophile attacking the electrophile to form the transition state that evolves into product (SN2 reaction mechanism = common in organic chemistry).

If the concentration of the nucleophile shows no effect on the rate of reaction, the reaction is SN1 reaction mechanism.

Both reaction mechanisms show a rate dependence on the electrophile concentration.

Reaction Order

There are 3 reaction orders: zero, first and second.

Zero order reaction proceeds at a constant rate, independent of reaction concentration.

Which of the following options is the rate law for this reaction, assuming that this reaction mechanism is correct?

A. k (PNO2) (PCO) B. k (PNO2)2 (PCO) C. k (PNO2)2 D. k (PNO2)

Answer: C

Solution: The slowest step in the reaction mechanism is the rate determining step. The rate depends on the two molecules of NO2 gas and CO has no effect on the rate of this reaction which means the addition of CO does not increase the rate of reaction.

Collision Theory

Reaction – reactants particle must collide with one another with sufficient energy to break chemical bonds in the reactants to form product.

Activated complex – a very high energetic and highly unstable species is formed.

Chemical reaction – is the effective collisions of reactant particles.

Reaction rate – is the measurement of the frequency of effective collisions.

Activation energy (Ea) – minimum energy required to break the chemical bonds in the reactant molecules and overcome the repulsion forces of the reactants molecules.

Energy profile / Reaction profile – the difference in energy between the reactants and activated complex.

Solution: Changing temperature changes the reaction rate. Adding a catalyst (usually positive catalyst) lowers the activation energy which results increases the reaction rate. Increasing the volume reduces the concentration of all reagents (include the reactants in the rate determining step). Increasing volume of a gas phase reaction results in a decreased in reaction rate. Therefore, if the reactant being added is not involved in the rate determining step, then it does not influence the rate.

Rate Constant and Rate Law

Rate constant is determined by the Arrhenius constant (collision orientation and frequency take into account)

k = A e –Ea/RT

where Ea is the activation energy, R is the molar gas constant, T is the absolute temperature and A is the frequency factor.

ln k = ln A – (Ea/RT) lg k = lg A – (Ea/2.303R) (1/T)

A plot of lg k against 1/T will be a linear graph with a slope of – (Ea/2.303R) and an intercept of lg A

they are transition metals, the oxides of transition metals and the oxides of aluminium and silicon

adsorption process which involve formation of bonds between the reactant molecules and the atoms on the surface (active sites) of the catalyst (solid metal)

4 steps in heterogeneous catalysis: reactant molecules are adsorbed on the surface, reactant molecules diffuse along the surface, reactant molecules react to form product molecules and molecules of product desorb from the surface

Welcome to ViNz CHaMaKh’s BLoG

Thank you for your willingness to come to this blog. This blog is my created as one of my small contribution, especially to my fellow classmates and friends. Many methods I have used to gain information and notes to publish it here and hey it's not easy! I hope a bit of knowledge can be shared here. For any rashness please forgive.Thank you for your support.